Pioneering ecologist Joseph Grinnell in 1914 began a seven year survey of the animals living in Yosemite National Park in California. Even then, human impacts such as the transformation of the Central Valley into an agricultural oasis were changing the landscape and the animals who lived there.

Nearly a century later, one cause for the transformation of California wildlife has come to overshadow all others: global warming. Now scientists have found that a rise of 6.7 degrees Fahrenheit (3.7 degrees Celsius) in average nighttime low temperatures (since 1920 when Grinnell concluded his research) is causing mammals in Yosemite to get a move on.

Evolutionary biologist Craig Moritz, director of the Museum of Vertebrate Zoology (founded by Grinnell) at the University of California, Berkeley, and his colleagues traced Grinnell's footsteps across the mountainous Yosemite terrain. Their inspiration: a quote on the wall of the museum from a 1910 paper authored by Grinnell in which he noted that future ecologists would be able to use the data he collected to determine how the same were faring a century later. They found that 16 of the 28 species documented by Grinnell in Yosemite had moved as far as 3,280 feet (1,000 meters) up the slopes in search of suitable climes—and that at least three, including the alpine chipmunk unique to California, had edged closer to extinction.

"The communities [of animals] are going to be very different—which species co-occur with which species," Moritz says. "That's already very clear. We can see that now."

The researchers measured populations using updated trapping methods, such as yogurt cups to lure voles and shrews, at 40 of the same sites where Grinnell camped nearly a century ago; in cases in which it was impossible to determine exactly where he had collected his specimens, "we sampled in similar habitats at the same elevation," says team member and environmental scientist Steven Beissinger of U.C. Berkeley.

The scientists found that fast-breeding species such as the California vole and California pocket mouse expanded their ranges by colonizing higher ground that was no longer as cold at night. Those animals such as Allen's chipmunk and the bushy-tailed woodrat already living on the peaks persisted in smaller areas at the highest extent of their ranges but their habitat had shrank. The alpine chipmunk now lives only 9,600 feet (2,925 meters) above sea level—an upward shift of nearly 2,000 feet (610 meters) from Grinnell's day—leaving it little leeway if the climate continues warming to move farther up without slipping off the mountainside entirely.

"I would want to monitor some of these species over the relatively short-term future, maybe every 10 years, to determine if the trend we see is continuing," says biologist James Patton of U.C. Berkeley, who also participated in the survey.

It remains unclear, however, exactly how these tiny mammals will fare if the average temperature continues to climb at its current rate. As it stands, the relative diversity of species in any given spot remains unchanged and closely related species in some cases showed very different responses—some declining and some thriving. And other species demonstrated an ability to change preferred habitats. "The most surprising thing was finding [the] pinyon mouse up in the High Sierra," Moritz says about the one-ounce (28-gram) mouse, which researchers found had jumped from its namesake pinyon pine habitat to higher elevation forest dominated by white bark pines. "As one of my colleagues put it: 'What the hell are you doing here?' That was the first indication that something big might be going on."

Though climate change is the most likely suspect, researchers fell short of proving it is behind all these habitat shifts given some puzzling findings. For instance, "pika are strongly affected by high temperatures but the average high temperatures haven't changed," Moritz notes. Nevertheless, the pika's range has been diminished. "We can't connect the pika physiology to the observed change yet."

These modern-day researchers hope that future scientists will find their work as helpful as they found the 4,000 specimens, 3,000 pages of field notes and 700 photos left behind by Grinnell. But it is already clear that turning the Yosemite area into a protected National Park in 1890 has permitted species to adapt to this rapid shift in climate by moving through relatively undisturbed habitats. "What we've looked at so far is a best-case scenario because we're looking at a protected landscape," Moritz says. "Community reorganization is a natural phenomenon but perhaps not at this pace. Can these species adjust to co-occur or not? I don't know. It's a grand experiment that I sort of wish wasn't happening."